This disclosure relates to a method for estimating State of health, SOH, of battery in a hybrid vehicle. Further this disclosure relates to hybrid vehicles with high power components (100 kW) where the power variations are large and fast. The disclosure is particularly advantageous in the field of electrical storage system for heavy vehicles, such as trucks, buses, construction vehicles, or the like.
Knowing the amount of energy left in a battery compared with the energy it had when it was full give the user an indication of how much longer a battery will continue to perform before it needs to recharge. This amount of energy is denoted State-of-Charge or SOC. SOC is the available capacity of a battery expressed as a percentage of its rated capacity. Batteries of hybrid vehicles have a voltage vs. State-of-charge characteristics, as shown by the voltage vs SOC curve in FIG. 1. The useful range denoted SOC window, of the battery is primarily in the “flatter” region of the characteristics.
The SOC window shows the available range of operation and it decreases from the ageing. An ageing battery changes its characteristics gradually so that it becomes more and more difficult to recharge. The characteristic alters slightly with aging which is shown in FIG. 1 and this alteration may be characterised in a parameter denoted State-of-Health or SOH. SOH is a measurement that reflects the general condition of a battery and its ability to deliver the specified performance compared with a new and fresh battery. The SOH parameter takes into account such factors as charge acceptance, internal resistance/impedance/conductance, capacity, electrical energy, self-discharge, ability to accept a charge and number of charge-discharge cycles. The consequence of attempting to charge a more and more aging battery is that the voltage increases at lower SOC level. This can lead to triggering of high voltage watchdogs and fault codes which can cause mission stop and/or be harmful to components in a hybrid vehicle. Therefore it is important to estimate the condition, SOH, of a battery and thus being able to gradually reduce the SOC window of the battery.
Unlike the SOC which can be determined by measuring the actual charge in the battery there is no absolute definition of SOH. SOH is a subjective measure in that different users derive the SOH from a variety of measurable battery performance parameters. SOH is a figure of merit of the condition of a battery (or a cell, or a battery pack), compared to its ideal conditions. The units of SOH are percent points where 100% corresponds to the battery condition matching the battery specifications. Typically, a battery SOH will be 100% at the time of manufacture and will decrease over time and use.
During normal operation it is difficult to estimate the SOH. If overestimated it may cause unnecessary watchdog fault triggers and mission stop, and if underestimated it may cause unnecessary loss of performance and premature scrapping of battery.
It is desirable to provide a method for estimation and calibration of State-of-health, SOH, of a battery in a hybrid vehicle, a method when the previously mentioned problems is at least partly avoided.
A method is provided according to an aspect of the present invention for estimation of State-of-Health, SOH, characteristics of a battery in a hybrid vehicle comprising a battery management unit, comprising the steps of:                calibrating the battery management unit b determining the present state-of-charge (SOC) window of the battery, comprising the steps of:        charging the battery while measuring the derivative of the open circuit voltage (OCV) with respect to the state-of-charge, dOCV/dSOC, continuously;        determining the first SOC level when the dOCV/dSOC is more than two times higher than a minimum dOCV/dSOC;        charging the battery by a first high charge current impulse during a predetermined time period;        discharging the battery while measuring the derivative dOCV/dSOC continuously;        determining the second SOC level when the derivative dOCV/dSOC is increasing;        discharging the battery by a second high current impulse during a predetermined time period;        determining the present SOC window of the battery by using the determined first and second SOC levels; and        estimating the SOH characteristics of the battery by comparing the determined present SOC window with a standard SOC window.        
In this context, the first SOC level is determined when the derivative dOCV/dSOC is more than two times higher than a minimum value of dOCV/dSOC, which minimum value occurs at the central plateau of the voltage vs SOC curve in FIG. 1. Similarly, the second SOC level is determined when the derivative dOCV/dSOC is increasing, i.e. becomes greater than the minimum value of dOCV/dSOC.
Both during charging and discharging steps, the battery is charged/discharged by a high current impulse. This current should be high relative to the capacity of the battery. In the field of battery charging, some battery charging constructions supply a relatively small magnitude current to the batteries, for example, a C/10 value where the symbol C is defined to be the one-hour capacity of a battery. Other battery charging constructions supply a significantly higher current to the batteries, such as, for example, a C value (C=1). In this context, a current impulse of C>1 would conventionally be considered to be high. When performing a discharge of the battery it is an advantage to use the internal combustion engine of the vehicle for achieving a sufficiently high current impulse.
As indicated above, the determined present SOC window is compared with a standard SOC window. The standard SOC window can, for instance, be a detected initial SOC window or a known original SOC window supplied by the manufacturer of the battery.
The invention, according to an aspect thereof, further relates to an alternative method for estimation of State-of-health (SOH) characteristics of a battery in hybrid vehicles, comprising the steps of:                charging and discharging the battery at least one time within an upper region of a State-of-charge (SOC) window, wherein the battery:        i) is charged to a first predetermined level in the upper region of the SOC window during a first time period;        ii) is charged by a first charge current impulse for pushing the SOC level of the battery to a level above the first predetermined level, during a second time period;        iii) is discharged by an electrical machine to a second predetermined level within the SOC window;        charging and discharging the battery at least one time within a lower region of the SOC window wherein the battery:        i) is charged to a third predetermined level in the SOC window, during a third time v) period;        ii) is discharged by an electrical machine to a fourth predetermined level in the SOC window;        iii) is discharged by a second current impulse, for pushing the SOC level of the battery to a level below the fourth predetermined level, during a fourth time period;        calibrating a battery management unit comprised in the hybrid vehicle by using the reached levels outside the SOC window;        estimating the SOH characteristics of the battery during the charge and discharge periods by using the battery management unit,        wherein; the first and third time period is longer than the second and fourth time period respectively; and the first predetermined level represents a higher voltage, than the second predetermined level and the third predetermined level represents a higher voltage, than the fourth predetermined level.        
According to an aspect of the invention the determined present SOC window is compared with a standard SOC window. The standard SOC window can, for instance, be a detected initial SOC window or a known original SOC window supplied by the manufacturer of the battery. The SOH characteristics of the battery can then be estimated by the outcome of the comparison between the determined present SOC window and the standard SOC window.
As SOH does not correspond to a particular physical quality, there is no consensus in the industry on how SOH should be determined. In the art it is known that the battery management unit may use any of the following parameters (singly or in combination) in combination with the inventive parameter to derive an estimated value for the SOH:                Internal resistance/impedance/conductance        Capacity        Electrical energy        Self-discharge        Ability to accept a charge        Number of charge-discharge cycles        
In addition, the battery management unit can define an arbitrary weight for each parameter depending on its contribution to the SOH value. By comparing one or more such measured (and possibly weighted) parameters and the determined SOC window with stored parameters and a known original SOC window the SOH characteristics of the battery can be estimated. The inclusion of one or more additional, known parameters can provide a more accurate estimate of the current state of health of the battery.
Some embodiments of the invention provide for a method for an improved estimation of the State-of-health of a battery.